Global urban population exposure to extreme heat

Abstract

Increased exposure to extreme heat from both climate change and the urban heat island effect-total urban warming-threatens the sustainability of rapidly growing urban settlements worldwide. Extreme heat exposure is highly unequal and severely impacts the urban poor. While previous studies have quantified global exposure to extreme heat, the lack of a globally accurate, fine-resolution temporal analysis of urban exposure crucially limits our ability to deploy adaptations. Here, we estimate daily urban population exposure to extreme heat for 13,115 urban settlements from 1983 to 2016. We harmonize global, fine-resolution (0.05°), daily temperature maxima and relative humidity estimates with geolocated and longitudinal global urban population data. We measure the average annual rate of increase in exposure (person-days/year^-1) at the global, regional, national, and municipality levels, separating the contribution to exposure trajectories from urban population growth versus total urban warming. Using a daily maximum wet bulb globe temperature threshold of 30 °C, global exposure increased nearly 200% from 1983 to 2016. Total urban warming elevated the annual increase in exposure by 52% compared to urban population growth alone. Exposure trajectories increased for 46% of urban settlements, which together in 2016 comprised 23% of the planet's population (1.7 billion people). However, how total urban warming and population growth drove exposure trajectories is spatially heterogeneous. This study reinforces the importance of employing multiple extreme heat exposure metrics to identify local patterns and compare exposure trends across geographies. Our results suggest that previous research underestimates extreme heat exposure, highlighting the urgency for targeted adaptations and early warning systems to reduce harm from urban extreme heat exposure.

Keywords: climate change; hazards; public health; sustainability; urbanization.

Fig. 1.

Global urban population exposure to extreme heat, defined by 1-d or longer periods when WBGT_max > 30 °C, from 1983 to 2016 (A), with the contribution from population growth (B), and total urban warming (C) decoupled.

Fig. 2.

(A) Municipality-level increase in the rate of urban population exposure to extreme heat from 1983 to 2016 and (B) the rate of increase in the total number of days per year when WBGT_max > 30 °C. (C) The share of population versus total urban warming in the rate of increase of total population exposure using WBGT_max > 30 °C. SI Appendix, Fig. S4 zooms in on Southern India, Ganges Delta, Nile river valley and delta, and Tigris–Euphrates river valley. Note, the largest increase in exposure (A) and days per year WBGT > 30 °C (B) are rendered last for emphasis. In C, urban settlements with a greater contribution from total urban warming (e.g., pink) are rendered last for emphasis.

Fig. 3.

Aggregated by region, the comparative contribution to the increase in the rate of urban exposure to extreme heat due to population growth versus total urban warming largely follows regional-level urban population growth rates as shown by the examples of (A) Latin America and the Caribbean, (B) Western Asia, (C) Southern Asia, and (D) Sub-Saharan Africa.

Fig. 4.

Two examples—Kolkata, India, in 1998 (A and B) and Aleppo, Syria, in 2010 (C and D)—of previously poorly or undocumented documented urban heat waves that our analysis uncovered. In both cases, the contrast between daily HI_max (A and C) with WBGT_max (B and D) estimates shows that while HI was not designed to be accurate at values HI_max > 50 °C, WBGT_max does not capture the amplitude of daily extremely hot temperature–humidity combinations.